The invention described herein was made in the course of work under a grant or award from the National Science Foundation.
This invention relates to catalytic chemical and biochemical conversion reactions and, more particularly, to novel methods and apparatus for effecting energy-efficient coupling of such conversion reactions with various energy-consuming post-conversion operations.
In carrying out the various catalytic chemical and biochemical conversion reactions, the post-conversion product purification operations of separating, concentrating and recovering the desired end products from reaction mixtures containing unconverted reactants, undesired reaction byproducts, inerts, and catalysts, typically require an investment in plant which exceeds that associated with the reactor itself. Furthermore, an external source of energy is generally needed to accomplish these physical processes of product purification and recovery.
These factors present a particularly serious economic obstacle to the commercial application of the new recombinant DNA-based biotechnology requiring the purification and concentration of the products (e.g., antibiotics) of enzyme- or whole cell-catalyzed reactions from dilute aqueous fermentation broths, in that the cost of product recovery from such reaction mixtures can often be prohibitive.
The enormous expense involved in product purification and recovery from varied dilute fermentation broths, is partly explained by thermodynamics, since the isothermal reversible work required for recovery of the pure material from a mixture is roughly proportional to the logarithm of the reciprocal of the concentration in the mixture in which the substance is found or produced. For example, until recently, interferon was painstakingly recovered from blood, where it is present at an effective concentration of order 10 ppb by weight (approximately 10.sup.-11 mole fraction). Its estimated production cost by this method of 10-20 Billion Dollars per pound accurately reflects the difficulty of its purification. Yet another example of the central importance of separation costs in the bioprocessing arena is provided by biomass-derived ethanol, the net energy yield and economic viability of which depends largely on the development of efficient separation processes to replace distillation for alcohol recovery.
In many nonbiological catalytic chemical conversion processes, as well, the costs of product separation and concentration often determine whether or not a process is economically feasible. The conventional product separation and concentration techniques, such as distillation and crystallization, require one or more separate vessels in addition to the catalytic reactor, and in a typical chemical process, amount for 70 per cent of the capital investment and 80 per cent of the energy costs.
Another cost-limiting energy-consuming post-conversion operation frequently encountered in catalytic chemical or biochemical conversion processes, is the requirement for a second-stage catalytic conversion reaction in order to obtain the desired end product from an intermediate precursor thereof formed as the product of a first stage catalytic conversion reaction. Multi-stage catalytic conversion processes are the rule rather than the exception in biochemical systems, and conversion of raw materials to final products often occurs by a sequence of catalytic reactions in industrial chemistry as well. A commonly encountered process of this type is an overall thermodynamically favorable multi-stage catalytic conversion process comprising a first-stage catalytic conversion of a reactant to an intermediate precursor of the desired end product by means of a substantially irreversible thermodynamically favorable reaction, followed by a second-stage catalytic conversion of the intermediate precursor to the desired end product by means of a reversible thermodynamically unfavorable reaction. Despite the fact that the overall conversion of the initial reactant to the desired end product is thermodynamically favorable, the yield of the desired end product in a conventional reactor containing the first and second-state catalysts cannot exceed the small, equilibrium-limited conversion of the intermediate precursor to the desired end product. The resulting product stream would consist of a dilute solution of desired end product in more concentrated intermediate. This mixture would require separation of the end product from the intermediate, followed by recycle of the latter for further conversion. Associated with these steps are requirements for costly process equipment and energy consumption which might tend to render the process economically unfeasible.
Engineering research on membranes and membrane processes has been directed toward two primary objectives. The first, and by far the more extensively investigated, is the use of semipermeable membranes for separation purposes in so-called "extractive reaction" schemes, where the purpose of the membrane is to selectively remove the product of a reversible reaction from the reaction zone. The second, and more recent, membrane research objective is the use of membranes as solid supports for immobilizing otherwise soluble enzymes and homogeneous catalysts. Two-layer composite membrane structures combining a semipermeable membrane layer with a catalytic membrane layer, have previously been described as a wrap for an ion-specific electrode in analytical applications (Blaedel, et al., Analytical Chemistry, Vol. 47, No. 9, pages 1602-1608, August, 1975); and for enhancing the flux of a permeant (Tanny, et al., Journal of Membrane Science, Vol. 4, pages 363-377, 1979). However, these prior art applciations of such multilayer composite membranes have not involved catalytic conversion reaction flow systems enabling a catalytic conversion reaction to be advantageously coupled either with a post-conversion product separation, recovery and concentration scheme, or with a post-conversion second-stage catalytic conversion reaction in a multistage conversion reaction scheme.